Where a hologram is to be displayed it can be important to minimize the distances from the hologram to the hologram's light source, while at the same time minimizing the intrusion of the light source into the viewing space. This is especially so when the light source is to be contained in an enclosure together with the hologram, and where it is desirable or necessary to limit the size of the enclosure.
For a substantially planar hologram, minimization of these distances substantially parallel to the hologram minimizes the width of the enclosure. Minimization of the distance substantially perpendicular to the hologram minimizes the depth of the enclosure. For a 600 millimeter×600 millimeter square planar hologram the minimum distance to the light source with present technology is approximately 1400 mm, and it will readily be appreciated that an enclosure depth of 1400 mm is a severe limitation on the display embodiments.
It is possible to reduce this depth by reflecting the light from the light source through mirrors, but there are significant limitations on the enclosure depth that still arise, and with present technology with a 600 mm square planar hologram, while illuminating all of the hologram, the minimum enclosure depth that results is 854 mm, which is still a severe limitation on the display embodiments.
This present limitation on the minimum enclosure depth arises because of certain characteristics of a hologram, which can be illustrated by reference to a planar reflection hologram as shown in FIG. 1. In FIG. 1, there is a planar hologram 10 (shown from the rear), that is viewable through an observable angle (beta) within a nominal viewing plane 12. The hologram is illuminated by light along a path 14. The illumination angle is defined as the angle between the light striking the hologram at a point and the normal (N) to the hologram at that point. There is an optimum illumination angle (alpha) between the incoming light to the hologram and the normal to the hologram where the light strikes the hologram, which gives optimum reconstruction of the light from the hologram.
For example, using parallel light striking the center of the hologram, the angle (alpha) might typically be 50 degrees from the normal to the center of the hologram, as shown in FIG. 1. It is possible to vary the illumination angle by plus or minus a certain number of degrees before the image produced disappears or suffers unacceptable distortion due to destructive interference of the wavefronts. The acceptable degree of variation might typically be 25 degrees, so that the actual illumination angle could vary between 25 to 75 degrees from the normal to the center of the hologram, while still producing an acceptable viewable image.
The range of acceptable illumination angles is a function of the grating spacing and thickness of the holographic film. A lower film thickness allows a greater illumination angle, but as the film thickness decreases, the diffraction efficiency decreases and because of this the brightness also decreases. There is therefore a practical limit that is reached. The grating spacing is determined by the wavelength of the light that is to be reconstructed, and hence, practically, by the need to view at least the primary colors red, blue and green. This is true even in the case of viewing a black and white hologram.
As the illumination angle is varied, it is possible to have the reconstructed light from the hologram show a different scene, forming an animated hologram. To avoid smearing of (or cross talk between) scenes of the animation, each scene must occupy a subtended angle which may be no less than a certain value. This dictates that there are a certain maximum number of different scenes that may be viewed by varying the illumination angle. This subtended angle is the angle subtended either by the lens of the eye at a given distance from the hologram in the case where the viewing plane is perpendicular to the plane in which the illumination angle is varied, or between the eyes on the viewing plane at a given distance from the hologram in the case where the viewing plane is parallel to the plane in which the illumination angle is varied. The subtended angle is much lower when the plane in which the illumination is varied is perpendicular to the viewing plane, and hence for a given variation in illumination angle, it is possible to have more scenes in this configuration. The different scenes may be arranged to be time-dependent views of the same scene and in this way varying the illumination angle gives rise to animation of the scene.
Because the number of scenes has a practical maximum, this means that the total amount of animation also has a practical limit. In either case it is desirable to be able to maximize the possible number of scenes, whether of animation or of static scenes.
Another limitation on enclosure size arises from the need to avoid having the light source in the field of view when viewing a reflection hologram through a viewing opening in the enclosure. The light source and all other physical parts must not be situated in the line of sight between the hologram and the viewing opening, but must stay outside or on an imaginary surface that includes all lines drawn from all points on the edges of the viewing opening to all points on the edge of the hologram, when viewed from each point on the edge of the viewing opening in turn. When the viewing opening and hologram are planar and parallel to one another, and the viewing opening is the same size as the hologram and is directly in front of the hologram, this surface consists of planes drawn from each side of the hologram coaxially with the perpendicular to the hologram. As shown in FIG. 2, in the case of the minimum illumination angle, the light source must be at least as far away as dimension D1 if it is not to encroach on the viewing opening. If the illumination angle is changed to 50 degrees, then the light source need only be distance D2 away from the hologram, but if this were the minimum illumination angle then the numbers of scenes viewable by altering the illumination angle would be decreased.
Even allowing for reflecting the light source through mirrors, with the above geometry, in the case of a 600 mm square hologram with a minimum distance from the center of the hologram to the light source of 1400 mm, to avoid intrusion of the light source into the viewing opening while still illuminating the complete hologram, the last mirror in the reflection chain may not be closer to the hologram than distance D1. This distance D1 is the minimum depth of any enclosure. With the above geometry, D1 is 854 mm which although less than the 1400 mm distance to the light source, still represents a severe limitation on the display embodiments.